The ejection head, also called simply ejector or injector in the following, according to the invention has characteristics that render it advantageous for use in numerous industrial sectors, even with specifics, characteristics and problems differing completely from one sector to the next.
In particular, among the possible sectors of application are, purely by way of example, that of ink jet printing, or that of fuel injection in an internal combustion engine.
As will be clear in the remainder of the description, the ejection head of the invention presents significant similarities, both structural and operational, with a thermal ink jet printhead, of the type working on the basis of the so-called bubble ink jet printing technology. Printheads of this type are widely known in the sector of ink jet printing technologies, where they are applied in a variety of solutions, and are still undergoing significant developments.
Therefore, for the sake of completeness and in order to facilitate the understanding of this description, and also in consideration of the fact that the ink jet printing sector constitutes, as already said, one of the possible and main fields of application of this invention, the general characteristics of these bubble type thermal ink jet printheads and some of their most recent developments will be set down in short below. As is known, in the printheads working with the bubble type ink jet technology, the ink contained in the printhead is brought to boiling point by thermal actuators consisting of electrical resistances which are powered with opportune current pulses in order to activate, inside the ink, the appearance of a bubble of vapour which, by expanding, causes ejection of the droplets through a plurality of nozzles in the printhead.
The printheads operating with the bubble technology may be divided into two main categories, depending on their structure, called respectively “top shooter” and “edge shooter”. In the first type, the nozzle consists of an aperture arranged immediately above the thermal actuator and separated from the latter by a small chamber filled with ink, so that the expansion of the bubble of vapour is used in a direction perpendicular to the thermal actuator so as to eject the droplet through the aperture. In the second type, the thermal actuator is disposed along the wall of a duct a short way from the duct's outlet section to the outside, so that the expansion of the bubble of steam is used in a direction transversal to the actuator to eject the drop laterally through the outlet section of the duct.
This bubble technology has been a standard in the printing sector for many years now, and is applied with success on numerous models of ink jet printheads, both for black and white printing and for colour printing. In particular, the ink jet printheads that work according to this technology are moving towards ever greater levels of integration and complexity, the objective being to comprise a greater number of circuits, nozzles and functions, and therefore attain ever greater printing speeds and definitions. One of the most recent examples of this technical development is represented by what are known as the monolithic printheads, i.e. by thermal ink jet heads in which the nozzle plate is made, not as a separate part, but together with the other parts of the printhead, particularly with those parts that constitute the driver circuits of the actuators and the hydraulic network for conveying the ink inside the printhead.
Therefore in these monolithic heads, the nozzle plate does not constitute a piece which is made separately and mounted at the end of the process of manufacturing the printheads, but rather a part which is formed progressively in the manufacturing process, so that each printhead acquires a typically monolithical structure integrating the various parts.
Hand in hand with the constant evolution of the bubble ink jet thermal printheads, the inks that can be used on these heads have also evolved considerably, which has led to a continuous improvement in their quality and reliability.
Generally speaking, evolution of the printheads has been accompanied by a corresponding evolution of the inks, the objective being to research ever better combinations between the printing media intended for receiving the droplets of ink, the structural characteristics of the head, and the chemical characteristics of the inks.
Typically this research into inks has been conducted with the objective of formulating inks capable both of improving the print quality on an ever broader range of print media, and of mating optimally with the new structures of printheads brought out with time.
In this way, both black and coloured inks have been formulated capable of minimizing the problem of clogging of the nozzles, cause by sedimentation of the pigments contained in the inks, despite the ever more intense miniaturization of the printheads and the reduction of the diameter of the nozzles in order to obtain ever smaller droplets.
Additionally, the research has permitted to define optimal combinations between inks and materials used in manufacturing the heads, with inks and materials compatible with one another, i.e. capable of not triggering off undesired reactions, and of maintaining their nominal characteristics in time, so as not to have negative effects on the operation and reliability of the printheads. In particular, this research into, as stated, constantly improving the combination between inks, print media, and printheads, has obviously addressed the formulation of inks having a low or practically null degree of chemical aggressiveness, namely inks free of substances capable of aggressing, corroding and reacting with, even only minimally, the various materials employed in manufacturing the heads and wetted by the inks.
For instance, it was attempted to avoid those inks containing substances that could interact with the organic compounds usually employed in making the junctions between the parts of the head. However, in this way, recent research in inks has in fact resulted in a certain consolidation, regarding their use on printheads, of inks with a null or practically null level of chemical aggressiveness.
At the same time, the possibility was ignored of employing these printheads in combination with particular types of ink and/or in general liquids which, though widely applied and capable of giving optimal results in certain fields, including different from printing true and proper, possessed however characteristics of chemical aggressiveness incompatible with the structure of the printheads that were being developed, and in particular contained aggressive substances certainly capable of corroding them and compromising their operation in time.
Besides, as is easy to imagine, it could be very useful and advantageous to be able to dispose of a new ink jet printhead, of the type based on the bubble technology or also on other technologies, having the ability to work with inks, perhaps already employed with success in various applications, including different from printing on paper, but unfortunately containing corrosive and/or aggressive substances likely to damage in time the structure and the materials of the currently known bubble type thermal ink jet heads. In fact, in this way the application possibilities for these printheads could be considerably extended, considering the new properties, essential characteristics and performance advantages that these corrosive substances could confer on the inks employed with them. Unfortunately however, as said, in reality the known ink jet printheads do not have a structure capable of resisting corrosive agents that may possibly be present in the inks employed with the printheads, so that in this hypothetical case they would rapidly enter decay.
For example, as is known, inks known to be typically aggressive, containing for instance urea, and/or having a determined acidic PH, can certainly not be used on the current thermal heads, because they would surely damage the junctions and the gluing zones between the different layers comprising the structure of the head.
There are also sectors in the art, again completely different from that of ink jet printing and the relative printheads, in which it is necessary to eject liquids in the form of droplets, preferably also very small, and in which these liquids to be ejected are particularly aggressive from the chemical viewpoint, and at any rate have a composition incompatible with the structure of the currently known printheads.
An important one of these sectors, briefly hinted at above, is that of the injection of a fuel, such as diesel or petrol, in the combustion chamber of an internal combustion engine. In this sector, the solutions normally adopted for fuel injection are based on mechanical type injectors, which however have the disadvantage of not reaching a sufficient degree of miniaturization of the droplets, or to put it better, that degree of miniaturization which would allow a better and more precise dosage of the fuel, and accordingly to attain better performance of the engine, such as for instance a higher thermal efficiency.
Therefore, potentially at least, this sector could avail of the ink jet technology which, in comparison with the traditional fuel ejectors, has been shown capable of obtaining droplets of liquid much smaller in volume, as also of obtaining in general a better and more efficient control of the quantity of liquid ejected in droplet form.
Yet another sector where there may be the need to dose in a precise and controlled way particularly aggressive liquids from the chemical viewpoint is the biomedical sector.